[Python-Dev] When does `PyType_Type.tp_alloc get assigned to PyType_GenericAlloc ?

[Randy Eels]

On Sun, Feb 7, 2016 at 8:45 PM, Guido van Rossum <guido at python.org> wrote:

> I think it's probably line 2649 in typeobject.c, in type_new():
>
>     type->tp_alloc = PyType_GenericAlloc;


I pondered it but it doesn't seem to be that. Isn't `type_new` called
*after* PyType_Type.tp_alloc has been called? I thought that line was only
being executed for user-defined types, and maybe some built-in types, but
certainly not PyType_Type.

On Sun, Feb 7, 2016 at 9:27 PM, eryk sun <eryksun at gmail.com> wrote:

> On Sun, Feb 7, 2016 at 7:58 AM, Randy Eels <randyeels at gmail.com> wrote:
> >
> > Yet, I can't seem to understand where and when does the `tp_alloc` slot
> of
> > PyType_Type get re-assigned to PyType_GenericAlloc. Does that even
> happen?
> > Or am I missing something bigger?
>
> _Py_InitializeEx_Private in Python/pylifecycle.c calls _Py_ReadyTypes
> in Objects/object.c. This calls PyType_Ready(&PyType_Type) in
> Objects/typeobject.c, which assigns type->tp_base = &PyBaseObject_Type
> and then calls inherit_slots. This executes COPYSLOT(tp_alloc), which
> assigns PyType_Type.tp_alloc = PyBaseObject_Type.tp_alloc, which is
> statically assigned as PyType_GenericAlloc.
>
> Debug trace on Windows:
>
>     0:000> bp python35!PyType_Ready
>     0:000> g
>     Breakpoint 0 hit
>     python35!PyType_Ready:
>     00000000`6502d160 4053            push    rbx
>     0:000> ?? ((PyTypeObject *)@rcx)->tp_name
>     char * 0x00000000`650e4044
>      "object"
>     0:000> g
>     Breakpoint 0 hit
>     python35!PyType_Ready:
>     00000000`6502d160 4053            push    rbx
>     0:000> ?? ((PyTypeObject *)@rcx)->tp_name
>     char * 0x00000000`651d8e5c
>      "type"
>     0:000> bp python35!inherit_slots
>     0:000> g
>     Breakpoint 1 hit
>     python35!inherit_slots:
>     00000000`6502c440 48895c2408      mov     qword ptr [rsp+8],rbx
>                                             ss:00000000`0028f960={
>                                             python35!PyType_Type
>                                             (00000000`6527cba0)}
>
> At entry to inherit_slots, PyType_Type.tp_alloc is NULL:
>
>     0:000> ?? python35!PyType_Type.tp_alloc
>     <function> * 0x00000000`00000000
>     0:000> pt
>     python35!inherit_slots+0xd17:
>     00000000`6502d157 c3              ret
>
> At exit it's set to PyType_GenericAlloc:
>
>     0:000> ?? python35!PyType_Type.tp_alloc
>     <function> * 0x00000000`65025580
>     0:000> ln 65025580
>     (00000000`65025580)   python35!PyType_GenericAlloc   |
>     (00000000`650256a0)   python35!PyType_GenericNew
>     Exact matches:
>         python35!PyType_GenericAlloc (void)
>

This makes quite a bit of sense. I completely overlooked the interpreter
init routines.

Thank you both Guido and eryk!
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